Fix utest-led-json.cpp should not depend on use-json

- The intent behind this commit is to fix the utest-led-json.cpp
  should not depend on use-json, because use-json is for production
  code and it should not affect test code.

Tested: built phosphor-led-manager successfully and Unit Test passes.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ia19b68d0167fd8c690ad1b15d29c266e9e77d90b
1 file changed